The present invention relates to a secondary battery control circuit, and specifically, to a control circuit for reliably performing the charging and discharging of a lithium ion secondary battery.
In the past, as controls for the charging of a lithium ion secondary battery, a charging voltage control, a charging current control, a control which enables the charging only within a predetermined temperature range, and the like, have been incorporated in a battery charger. In a battery pack in which a lithium ion secondary battery, etc., is incorporated, a control which detects the voltage of a battery and interrupts the charging when the voltage is equal to or greater than a predetermined voltage value is provided as a provision for the case where charging voltage control does not work. The above-described charging interruption control provided in the battery pack is termed as overcharge protection.
Furthermore, a control has been performed in which the temperature of a battery in the above-described battery pack is measured by a thermistor, an output of which is connected to a battery charger, and in the battery charger, the charging is enabled only within a predetermined temperature range.
On the other hand, conventionally, as the control associated with the discharging of the lithium ion secondary battery, etc., a control which interrupts the discharging when a load short-circuit occurs, a control which detects an excessive rush current during the discharging and interrupts the discharging, and a control which detects a constant current during the discharging and interrupts the discharging have been performed. These controls are generically referred to as an overcurrent protection.
Controls at the battery pack side, such as the above-described overcharge protection, the thermistor control, and the overcurrent protection, are generically referred to as safety circuits.
The above-described overcurrent protection is one of the protection functions for a battery such as a lithium ion secondary battery, etc. However, in many cases, in preparation for a case where this protection does not work, a PTC element having a ring shape is installed in the lithium ion secondary battery itself, or a PTC element is provided in the battery pack.
The PTC (Positive Temperature Coefficient) element is an element such that the resistance value is low during a normal operation, but abruptly increases when heated by itself or by an ambient temperature. This element has been used for overcurrent/heatup protection.
A battery pack which uses a lithium ion secondary battery and the secondary battery control circuit (safety circuit) is described in, for example, Japanese Laid-Open Publication No. 10-275612.
However, in the above-described conventional structure, in the case where a PTC element having a ring shape which has been installed in a lithium ion secondary battery itself is activated, an expansion force in a direction of the thickness of the PTC element to affected in a direction in which a caulking portion of a battery sealing portion is opened. This has been a cause for leakage of electrolytic solution. Furthermore, there has been a concern that the deterioration in a life cycle or the swelling of the secondary battery which results from the fully-charged lithium ion secondary battery, etc., being left in a high temperature condition.
Furthermore, there has been a concern that circuit malfunction, corrosion, or problems due to hydrofluoric acid generated by adding water to an electrolytic solution may be caused in the case where a liquid infiltrates into a secondary battery such as a lithium ion secondary battery or into a battery pack in which the above-described secondary battery is installed, or in the case where an electrolytic solution inside the battery is leaked.
In general, the main component of an electrolytic solution which is used for a lithium ion secondary battery is lithium hexafluorophosphate.
The chemical reaction caused in the case where water is infiltrated into lithium hexafluorophosphate is represented by expression (1-1). By this chemical reaction, hydrofluoric acid (i.e., a very strong acid) is generated.
xe2x80x83LiPOF6 (lithium hexafluorophosphate)+H2Oxe2x86x92LiPOF4.2HF(water is present)xe2x86x92LiF+POF3↑+2HF↑ (after decomposition)xe2x80x83xe2x80x83(1-1)
The present invention solves such conventional problems. An objective of the present invention is to provide a secondary battery control circuit which can reliably control charging/discharging without activating a PTC element, which reduces deterioration in life cycle and the occurrence of swelling of the battery, and which can be installed in a small space section in a battery pack.
In order to achieve a solution to the above problem, a secondary battery control circuit of the present invention includes: a temperature detection section for detecting a temperature of a secondary battery; and a control section for controlling charging/discharging of the secondary battery based on a temperature detected by the temperature detection section, wherein the control section interrupts charging of the secondary battery in a case where the temperature detected by the temperature detection section is out of a predetermined first temperature range, and interrupts discharging of the secondary battery in the case where the temperature detected by the temperature detection section is out of a predetermined second temperature range. Thus, a liquid leakage phenomenon of an electrolytic solution due to the activation of a ring-shaped PTC element installed in a lithium ion secondary battery itself can be prevented.
The predetermined second temperature range may include the predetermined first temperature range.
A secondary battery control circuit of the present invention includes: a voltage detection section for detecting a voltage of a secondary battery: a temperature detection section for detecting a temperature of the secondary battery, and a control section for controlling charging/discharging of the secondary battery based on a voltage detected by the voltage detection section and a temperature detected by the temperature detection section, wherein in a case where the voltage detected by the voltage detection section is equal to or greater than a predetermined first voltage value, and the temperature detected by the temperature detection section is equal to or greater than a predetermined temperature, the control section discharges the secondary battery until the voltage of the secondary battery reaches a predetermined second voltage value. Thus, the deterioration in life cycle and swelling of the battery which result from a fully-charged secondary battery, such as a lithium ion secondary battery, etc., being left in a high temperature condition can be prevented.
It is preferable that the predetermined first voltage value and the predetermined second voltage value are detected by a single circuit having a hysteresis.
It is preferable that either the predetermined first voltage value or the predetermined second voltage value is equal to an overcharge releasing voltage value.
A secondary battery control circuit of the present invention includes a control section for interrupting charging/discharging of the secondary battery in the case where a liquid is detected by a liquid detection section for detecting infiltration or generation of a liquid inside the secondary battery or inside a battery pack in which the secondary battery is installed. In such a structure, when circuit malfunction, corrosion, or problems due to hydrofluoric acid generated by adding water to an electrolytic solution (which may be caused in the case where a liquid is infiltrated into a secondary battery such as a lithium ion secondary battery or into a battery pack in which the above-described secondary battery is installed, or in the case where an electrolytic solution inside the battery is leaked) are caused, the charging/discharging of the secondary battery is disabled, whereby a user can be notified of the occurrence of a problem.
The secondary battery control circuit further includes a temperature detection section for detecting a temperature of the secondary battery, and the control section may control charging/discharging of the secondary battery based on a temperature detected by the temperature detection section.
In the case where the secondary battery control circuit is formed on a single semiconductor chip, a small sized secondary battery control circuit is achieved. By enclosing the semiconductor chip in a sealing section of the secondary battery or by mounting the semiconductor chip in a narrow space section of a battery pack, a secondary battery in which a secondary battery control circuit is installed or a small sized secondary battery pack in which a secondary battery control circuit is mounted in a narrow space section of a battery pack can be provided.
Hereinafter, functions of the present invention are described.
According to one aspect of the invention, in the case where a temperature detected by a temperature detection section is out of a predetermined first temperature range or out of a predetermined second temperature range, charging/discharging of the secondary battery is interrupted. This prevents a liquid leakage phenomenon of an electrolytic solution due to the activation of a PTC element installed in the secondary battery.
According to another aspect of the invention, in the case where a voltage detected by a voltage detection section is equal to or greater than a predetermined third first voltage value, and the temperature detected by a temperature detection section is equal to or greater than a predetermined fourth temperature, the secondary battery is discharged until the voltage of the secondary battery reaches a predetermined second voltage value which is smaller than the predetermined first voltage value. By such a forced discharge, deterioration in the life cycle and swelling of the secondary battery due to the charged secondary battery being in the high temperature condition can be prevented.
According to yet another aspect of the invention, in the case where the generation of a liquid is detected by a liquid detection section, charging/discharging of the secondary battery is interrupted. This allows a user to notice a malfunction or a corrosion of a circuit due to a liquid, such as water or an electrolytic solution, etc., or a problem caused due to hydrofluoric acid being generated by addition of water to an electrolytic solution.